What would happen to you if you drank seawater?

November 11, 2003

Dear Straight Dope:

Let's say you're alone on a raft in the middle of the South Pacific, with no fresh water to speak of. What are the health implications of drinking salt water--will it make you vomit, will it make you delirious, will it dehydrate you further, etc.?

— Jesse Sussell

Water, water, every where,
Nor any drop to drink.

--Samuel Taylor Coleridge, The Rime of the Ancient Mariner, part II, stanza 9

We need water to live, and we need salt to live, so what's the big deal about sailors not being able to drink saltwater?

The answer is that we only need a small amount of salt to live. According to the Salt Institute (a non-profit association of salt producers, founded in 1914), the recommended daily dose is around 500 mg/day--around a quarter of a teaspoonful. The optimal amount of salt varies based on the person's lifestyle, genetic makeup and geographic location (basically, all factors that affect how often and how much you sweat). Most Americans consume much more than they need, around 3500 mg/day.

Mark Kurlansky, in Salt: A World History, says that "salt is a chemical term for a substance produced by the reaction of an acid with a base. When sodium, an unstable metal that can suddenly burst into flame, reacts with a deadly poisonous gas known as chlorine, it becomes the staple food sodium chloride, NaCl, from the only family of rocks eaten by humans." That's common salt, the sort we sprinkle on food, which has a taste we call salty. Sodium chloride is essential for digestion and respiration. "Without sodium, which the body cannot manufacture, the body would be unable to transport nutrients or oxygen, transmit nerve impulses, or move muscles, including the heart," says Kurlansky.

But when we're talking about seawater, we're not just talking about common salt. Other compounds and elements and minerals called salts are found in ocean water, such as epsom salts, potassium salts, iodine salts, and so forth. Some of these taste bitter or sour, although they may be of value to the human diet, such as magnesium chloride and potassium chloride.

The composition of ocean salt is very complex. According to the U.S. government, salinity is measured by the concentration of dissolved salts. That is, we take the amount (by weight) of the salt in water, expressed as "parts per million" (ppm.) The government defines fresh water as having less than 1,000 ppm of dissolved salts--in other words, less than 0.1% of the weight of the water comes from dissolved salts. By contrast, human blood is around 0.9% salt, and about 0.25% of our total body weight is salt.

On this scale, the ocean is classified as "highly saline" (over 1.0% dissolved salts.) In fact, seawater is around 3.5% dissolved salts by weight. That's about three times as salty as human blood. That's way more salt than we can safely metabolize.

Interestingly, the proportion of minerals and salts in human tissue is very similar to the composition of seawater. The adult human body contains enough salt to fill about three salt shakers, but the salt is constantly lost through bodily functions like sweating, crying, urinating, etc. It is essential to replace this lost salt, but not to over-replace. We can't tolerate seawater consumption. Our cells can't take it and our kidneys can't take it.

Which gets us at last to your question--what happens if you drink seawater? Bill Bryson puts it vividly:

Take a lot of salt into your body and your metabolism very quickly goes into crisis. From every cell, water molecules rush off like so many voluntary firemen to try to dilute and carry off the sudden intake of salt. This leaves the cells dangerously short of the water they need to carry out their normal functions. They become, in a word, dehydrated. In extreme situations, dehydration will lead to seizures, unconsciousness, and brain damage. Meanwhile, the overworked blood cells carry the salt to the kidneys, which eventually become overwhelmed and shut down. Without functioning kidneys you die. That is why we don't drink seawater.

The U.S. government notes that there have been significant efforts at desalinizing seawater, notably in California and Florida, where a few towns are using desalinization methods to remove the salt from seawater and make it suitable for drinking. (For more information, see the Water Education Foundation website at wwwga.usgs.gov/edu/drinkseawater.html. The cost is high, around five times the cost of processing water from normal supply sources. Technology is improving and costs are dropping, though, and there is speculation that desalinization techniques could be used to bring water to irrigate the deserts of the Middle East, for instance.

How come some marine mammals can drink seawater and we can't? Robert Kenny of the University of Rhode Island at the Scientific American website says it's not certain that they actually do drink seawater. Marine mammals are mostly carnivorous, so their food supply has a salt content similar to their own blood. They can get most of their water through their food, without the need to drink seawater. Most sea-dwelling mammals produce very salty urine; their kidneys have adapted to enable them to unload excess salt in quantity. Ours haven't--our kidneys can make urine only slightly less salty than salt water. Hence, if you drink too much salt water, you need to urinate more water than you drank to get rid of the excess salt, and dehydration sets in. Drinking even a little seawater starts you down a dangerous road: The more you drink, the thirstier you get.

Bryson, Bill; A Short History of Nearly Everything; Broadway Books, NY (2003)

— Dex

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.